In prior art electrostatic drum printing on sheets, such as in facsimile machines, an analog signal was converted directly into a charge pattern. However, this usually meant that only a single raster line at a time was being painted onto a rotating drum. Attempts have been made to increase the rate of printing by increasing the speed of rotation of the drum. However, this sometimes results in a loss of resolution.
In U.S. Pat. No. 3,656,173 R. Fussel discloses a printing system involving creation of a latent electrostatic image on a sheet of paper carried by a rotating drum. A fine stream of liquid developer is directed onto the paper at a spaced apart distance from a stylus which deposits a charge pattern. A vacuum pickup head is positioned to follow the trace of the developer so that excess liquid may be removed from the paper surface. The drum is axially translated with respect to the developer stream, the stream tracing a helical pattern on the drum surface.
One of the chief benefits achieved in the drum printer described in the Fussel patent is that the sheet being printed upon is fully stabilized by the drum during the printing process. This is an advantage over web systems, where sheets being printed are subject to discernable changes in size. On the other hand, web systems allow very rapid scanning of the sheet past a stationary head for extremely fast printing. Line printers print the entire width of a sheet at such fast rates, but employ wide printing heads which are very expensive.
In U.S. Pat. No. 4,257,071 R. Lamb discloses a planar image scanning and printing system wherein a linear array of optical sensing elements is moved in a raster scanning pattern across a sheet. The linear array of scanning elements may be combined with a linear array of printing elements, sharing similar motion. The array of scanning printing elements allows sequential printing of digital words using a lateral scanning pattern analogous to the scanning of a cathode ray tube.
An object of the present invention was to devise an electrostatic printer having the sheet stability of a drum type printer, but also using printing elements which are faster than those in conventional facsimile systems, yet without the expense involved in full width line printer heads.
Another object of the invention was to devise a color electrostatic printer.
Yet another object of the invention was to devise a digital electrostatic printer with precise color dot registration.